1. Field of the Invention
This invention relates generally to a method of manufacturing a ferrite magnetic transducer head, and more particularly to a method of bonding two ferrite cores with glass material.
2. Description of the Prior Art
Recently, an oxide glass which is sufficient and stable in adhesive force and relatively low in melting point is widely used for a so-called gap melt bondage or injection of the effective gap of a ferrite magnetic head. This has the base that glass and ferrite are both oxides and mutual diffusion therebetween can be carried out sufficiently. However, this fact causes deteriorations in various characteristics in the surface layer of the ferrite which contacts with the glass. These deteriorations appear a great deal in a low melting point of glass whose components are modified oxides such as Na.sub.2 O, PbO, ZnO or Bi.sub.2 O.sub.3. That is, upon melt-bonding, the surface layer of ferrite is molten into glass or the glass erodes the surface layer of ferrite and, as shown in FIG. 1, a second phase 3 made of for example, .alpha. - Fe.sub.2 O.sub.3 or other non-magnetic material is formed in the boundary between glass 1 and ferrite cores 2. The formation of such phase 3 results in distortion of the shape of ferrite cores 2 which makes it impossible to manufacture ferrite parts of a predetermined design and also deteriorates the magnetic characteristics of the ferrite in its surface layer. That is, its permeability is lowered and its maximum magnetic flux density is also lowered, which is a serious deterioration of ferrite as a magnetic material. Since property of thermal expansion is different in ferrite and glass, when the ferrite is melt-bonded with glass, cracks may appear in the ferrite and the glass due to their shrinkage-distortions upon cooling.
FIG. 2 is a graph showing the thermal expansion characteristics of the glass and ferrite in which a curve I shows the characteristic of SiO.sub.2 - Na.sub.2 O-B.sub.2 O.sub.3 system glass and a curve II that of Mn-Zn ferrite. The distortion caused by the difference of the thermal expansion characteristics between the glass and ferrite may increase when the amount of used glass increases. As a result, cracks also increase to lower a great deal the yield of ferrite parts. Even in the case where no cracks are present, a residual strain or distortion is given in the ferrite which may deteriorate the magnetic characteristics of ferrite such as permeability, maximum magnetic flux density and so on.
An oxide glass having a relatively low melting point which is lower than ferrite in hardness, so that when ferrite cores 2A and 2B are melt-bonded with an oxide glass 1 of relatively low melting point to make a ferrite magnetic head 4, the glass 1 is first abraded to form a concave 1a when a magnetic tape (not shown) travels in contact with the head 4 as shown in FIG. 3. In this case, magnetic powders torn from the magnetic tape may be stored in the concave 1a, which is undesirable.
In order to avoid the above defects, by way of example, there is proposed in U.S. Pat. No. 3,188,400 that the ferrite head cores be melt-bonded at the temperature of 1000.degree.C-1250.degree.C with sintering glass containing particles which are the same as those forming the ferrite cores to form a transition layer such as ferrite solid-solution between the head cores and the glass, which transition layer improves the bonding between the head cores and glass. This method can improve the bonding between the ferrite cores, but since the transition layer is formed between the ferrite cores and glass, its effective gap becomes different from that determined initially by a spacer. Further, the ferrite is eroded by the glass from a microscopic point of view.